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Half of the energy emitted by late-T- and Y-type brown dwarfs emerges at 3.5< lambda um < 5.5. We present new L' (3.43 < lambda um < 4.11) photometryobtained at the Gemini North telescope for nine late-T and Y dwarfs, andsynthesize L' from spectra for an additional two dwarfs. The targets includetwo binary systems which were imaged at a resolution of 0.25". One of these,WISEP J045853.90+643452.6AB, shows significant motion, and we present anastrometric analysis of the binary using Hubble Space Telescope, Keck AdaptiveOptics, and Gemini images. We compare lambda ~4um observations to models, andfind that the model fluxes are too low for brown dwarfs cooler than ~700K. Thediscrepancy increases with decreasing temperature, and is a factor of ~2 atT_eff=500K and ~4 at T_eff=400K. Warming the upper layers of a model atmospheregenerates a spectrum closer to what is observed. The thermal structure of coolbrown dwarf atmospheres above the radiative-convective boundary may not beadequately modelled using pure radiative equilibrium; instead heat may beintroduced by thermochemical instabilities (previously suggested for the L- toT-type transition) or by breaking gravity waves (previously suggested for thesolar system giant planets). One-dimensional models may not capture theseatmospheres, which likely have both horizontal and verticalpressure/temperature variations.

3.8um Imaging of 400-600K Brown Dwarfs and Orbital Constraints for WISEP J045853.90+643452.6AB